Virtually every form of progressive heart failure (HF) is associated with increased fibrosis. Currently there is no approved therapy to specifically target myocardial fibrosis in the diseased heart. Until very recently, HF studies have been largely limited to cardiomyocytes, primarily due to unavailability of cardiac fibroblast (CF)-specific mouse models. Recently, we used novel CF-specific mouse models to demonstrate that deletion of CF-GSK- 3? is detrimental in the ischemic heart. This successful utilization of CF-specific gene targeting provides a unique opportunity to further employ these newly optimized models to understand the role of CFs and their activation in myocardial disease process in vivo. The long-term goal of the proposed studies is to identify new therapeutic targets for the treatment of myocardial fibrosis and subsequent HF. Three specific aims are designed to identify the key signaling pathways and underlying primary mechanisms responsible for regulation of myocardial fibrosis. Aim 1: To elucidate the molecular mechanism by which CF-GSK-3? regulates fibrotic remodeling in the ischemic heart. The GSK-3 family consists of two isoforms, ? and ?. In stark contrast to our recent report with GSK-3?, our preliminary studies suggest that CF-specific deletion of GSK-3? is protective, post-MI. Based on this observation we hypothesize that CF-GSK-3? is deleterious in the process of post MI fibrotic remodeling. This hypothesis will be tested by employing CF-GSK-3? KO (periostin-cre) and tamoxifen- inducible fibroblast specific GSK-3? KO (TCF21-cre) mouse models. Aim 2: To define the molecular mechanisms by which CF-specific deletion of GSK-3? leads to adverse myocardial fibrosis. We hypothesize that GSK-3?, SMAD-3 and ?-catenin function as an integrated central profibrotic signaling cascade. We will cross our GSK-3?fl/fl mice with SMAD-3fl/fl and ?-cateninfl/fl mice to determine whether inhibition of the SMAD-3 and ?-catenin axis is sufficient to abolish myocardial fibrosis in GSK-3?KO mice. Aim 3: Determine the mechanisms by which Integrin ?1?1 (ITG?1?1) cross-talks with the profibrotic SMAD-3 and p38 pathways and identify the role of this interaction in regulation of myocardial fibrosis. Our preliminary data suggest that mechanical stretch couples to SMAD-3 activation even in the absence of TGF-?1 treatment. The predominant integrin expressed in CFs is ITG?1?1. In this aim, we will test the hypothesis that ITG?1?1 negatively regulates the TGF-?1/SMAD-3 and p38 pathways and thus exerts a critical break on myocardial fibrotic remodeling. The proposed approach is innovative, because it departs from status quo by utilizing novel CF- specific loss of function mouse models and isolated cells from them to understand the molecular mechanism of myocardial fibrosis in diseased heart. New research horizons are expected to become attainable as a result. The proposed research is highly significant, since it proposes novel strategies to prevent fibrotic remodeling in diseased heart.